The present invention relates to a light-emitting-diode array and a light-emitting-diode element used in this light-emitting-diode array.
A conventional light-emitting diode (LED) array is disclosed in a publication, "Design of an Optical Printer" Triceps WS 6, 1985, pp. 121-126. The structure of the conventional LED array will be briefly described with reference to FIG. 6A and FIG. 6B.
In a conventional LED array 50, an n-type GaAsP layer 52 is formed on an n-type GaAs substrate 51, and a diffusion mask 54 having apertures is formed on the n-type GaAsP layer 52. P-type diffusion regions 56 are formed in the n-type GaAsP layer 52, by diffusion through the apertures of the diffusion mask 54.
P-type electrodes 58 are provided to extend on the surface of the diffusion mask 54 and the p-type diffusion regions 56. An n-type electrode 60 is provided on the under side of the substrate 51.
The diffusion regions 56 form pn junctions for emitting light and are arranged to form a row. Each chip is separated by dicing from a wafer. The p-type electrodes 58 are in contact with the corresponding diffusion regions 56.
The conventional LED array 50 encounters a limitation on the density of the diffusion regions, and hence light-emitting-diodes, because of the limitations on the density of the electrode pads.
(1) Where a high-density LED array of, for instance, 1200 DPI is formed, if the electrode pads (also called bonding pads) connected to the diffusion regions and the LEDs are disposed in one-to-one relationship, the density of the electrode pads becomes too high, and the electrode lines cannnot be formed on the surface of the n-type GaAsP layer 52.
(2) Where the density of the p-type electrodes becomes high, the size of the electrode pads must be reduced, and the connection between the electrode pads and the driver IC by means of bonding wires or the like becomes difficult.
Accordingly, it has been desired to realize an LED array permitting the diffusion regions to be formed at a higher density.
Another problem associated with the prior art is that the n-type GaAsP layer 52 and the substrate 51 are electrically connected.